1. Field of the Invention
The present invention relates to a DC-DC converter preferably for use in, for example, hybrid vehicles, and that is capable of determining a effective capacity or characteristic degradation state of a 12V lead-acid battery that is the same as that used in gasoline-driven vehicles in the related art.
2. Description of the Related Art
Repeated charging/discharging of secondary batteries degrades the internal physicochemical property of the secondary batteries and reduces the effective capacity of the secondary batteries. When the effective capacity of such a secondary battery decreases to a certain level, it is determined that the secondary battery reaches the end of its useful life. It is necessary to replace the secondary battery.
In order to prevent the full discharging of a lead-acid battery, that is, “battery exhaustion” in vehicles for starting an engine with a starter motor, it is important that the degree of reduction in the effective capacity of the battery is determined.
On the other hand, “hybrid vehicles” having both a gasoline engine and an electric motor are becoming increasing popular. In a region where the number of times of acceleration/deceleration is small and a fuel efficiency is high, for example, on an expressway, such a hybrid vehicle drives a gasoline engine, generates power using an electric motor, and stores the generated power in a secondary battery such as a lithium-ion battery. In a region where the number of times of acceleration/deceleration is large and a fuel efficiency is low, for example, on a general road, the hybrid vehicle travels by driving the electric motor with power supplied from the lithium-ion battery.
In general hybrid vehicles, a lithium-ion battery for supplying power to an electric motor, which allows a vehicle to travel, generates a voltage of approximately 300 V to 400 V. Like gasoline-driven vehicles, such a hybrid vehicle needs to have a lead-acid battery (whose output voltage is 12 V) for driving electric components and starting a gasoline engine.
In gasoline-driven vehicles in the related art, a method is employed of attaching an alternator (generator) to a gasoline engine and charging a lead-acid battery while the gasoline engine runs. In the case of hybrid vehicles, an electric motor is installed in advance and the installation of an alternator is therefore a waste of space. Accordingly, in hybrid vehicles, a method is employed of stepping down a voltage of approximately 300 V to 400 V generated by an electric motor to 12 V with a DC-DC converter and charging a lead-acid battery without an alternator.
However, when “battery exhaustion” of a lead-acid battery occurs, like gasoline-driven vehicles, hybrid vehicles cannot move. Even if a lithium-ion battery tries to start a gasoline engine, it is impossible to start the gasoline engine because a DC-DC converter does not have a current capacity required for driving of a starter motor. Thus, in hybrid vehicles, it is also important that the effective capacity or characteristic degradation state of a lead-acid battery is determined, that is, it is determined whether the lead-acid battery is at the end of its useful life or when the lead-acid battery reaches the end of its useful life.
Japanese Unexamined Utility Model Application Publication No. 2-45476 discloses an apparatus for measuring the effective capacity of a battery. The apparatus disclosed in Japanese Unexamined Utility Model Application Publication No. 2-45476 will be described with reference to FIG. 1. The apparatus includes a microcomputer 1, a charger 3 for charging a secondary battery 2, an A/D converter 4, a constant current load 5, a display device 7, a condition setting unit 6, and an external apparatus 8. The secondary battery 2 is connected to the charger 3 via a switch 10, and is connected to the constant current load 5 via a switch 11. The A/D converter 4 and the constant current load 5 are used to measure the discharging characteristic of the secondary battery 2 after charging has been completed. At the time of measuring the discharging characteristic, a constant current load is controlled in response to an instruction for a load current value transmitted from the microcomputer 1, the switch 11 is turned on, and the constant current load 5 is connected to the secondary battery 2. At that time, the switch 10 connected to the charger 3 is turned off.
Thus, a current of a predetermined value is passed through a load connected to a battery, and the fall time of a voltage across the battery (a time required for the voltage across the battery to decrease to a predetermined threshold voltage) is measured.
As disclosed in Japanese Unexamined Utility Model Application Publication No. 2-45476, in an apparatus in the related art for measuring the effective capacity of a secondary battery, it is necessary to provide a measurement circuit in addition to a charging circuit. This requires space and cost.